The present Invention relates to novel fluids and techniques to optimize/enhance the production of hydrocarbons from subterranean formations. To recover hydrocarbons (e.g., oil, natural gas) it is of course necessary to drill a hole in the subsurface to contact the hydrocarbon-bearing formation. This way, hydrocarbons can flow from the formation, into the wellbore and to the surface. Recovery of hydrocarbons from a subterranean formation is known as xe2x80x9cproduction.xe2x80x9d One key parameter that influences the rate of production is the permeability of the formation along the flowpath that the hydrocarbon must travel to reach the wellbore. Sometimes, the formation rock has a naturally low permeability, other times, the permeability is reduced during, for instance, drilling the well. When a well is drilled, a fluid is circulated into the hole to contact the region of the drill bit, for a number of reasonsxe2x80x94including, to cool the drill bit, to carry the rock cuttings away from the point of drilling, and to maintain a hydrostatic pressure on the formation wall to prevent production during drilling.
Drilling fluid is expensive particularly in light of the enormous quantities that must be used during drilling. Additionally, drilling fluid can be lost by leaking off into the formation. To prevent this, the drilling fluid is often intentionally modified so that a small amount leaks off and forms a coating on the wellbore, or a xe2x80x9cfiltercake.xe2x80x9d
Yet once drilling is complete, and production is desired, then this coating or filtercake must be removed. The present fluids and techniques are directed to removing this filtercake or other such damage in the wellbore and near-wellbore region, that results either intentionally (in the case of drilling fluid) or unintentionally (in the case of scale deposits from produced water or dewatered fluids from workover/stimulation operations performed on the well).
Conventional treatments for removing filtercake include: aqueous solution with an oxidizer (such as persulfate), hydrochloric acid solution, organic (acetic, formic) acid, combination of acid and oxidizer, and aqueous solutions containing enzymes. For instance, the use of enzymes to remove filtercake is disclosed in U.S. Pat. No. 4,169,818, Mixture of Hydroxypropylcellulose and Poly(Maleic Anhydride/Alkyl Vinyl Ether) as a Hydrocolloid Gelling Agent (1979) (col. 1, In. 42); U.S. Pat. No. 3,515,667, Drilling Fluid Additive (1970); U.S. Pat. No. 3,509,950, Well Drilling Mud and Screen Composition of Use Thereof; U.S. Pat. No. 2,259,419, Well Drilling (1941). Chelating agents (e.g., EDTA) are also used to promote the dissolution of calcium carbonate. See, C. N. Fredd and H. S. Fogler, Chelating Agents as Effective Matrix Stimulation Fluids for Carbonate Formations, SPE 372212 (1997); C. N. Fredd and H. S. Fogler, Alternative Stimulation Fluids and Their Impact on Carbonate Acidizing, SPE 31074 (1996), both articles are hereby incorporated by reference in their entirety. According to conventional teaching, the oxidizer and enzyme attack the polymer fraction of the filtercake; the acids mainly attack the carbonate fraction (and other minerals). Generally speaking, oxidizers and enzymes are ineffective in degrading the carbonate fraction; likewise, acids have very little effect on polymer.
In addition, numerous problems plague conventional techniques of filtercake removal. Perhaps the most troublesome is the issue of xe2x80x9cplacement.xe2x80x9d For instance, one common component in filtercake is calcium carbonate. The substance of choice to remove calcium carbonate is hydrochloric acid. Hydrochloric acid reacts very quickly with calcium carbonate. What happens then, is that the filtercake begins to dissolve, therefore dramatically increasing the permeability of the wellbore face, so that the wellbore region is no longer xe2x80x9csealed offxe2x80x9d from the formation. Once this happens, the entire clean-up fluid may then leak off into the formation through this zone of increased permeability (xe2x80x9cthief zones,xe2x80x9d or discrete zones within the interval of very high permeability where more filtercake dissolution has occurred than at other places along the interval).
A second problem with removal of filtercake is that it is comprised of several substances, and which are, as mentioned earlier, not generally removable with a single substance. Calcium carbonate and organic polymers (e.g., starch and other polysaccharide) are two primary constituents of conventional drilling fluids that form a filtercake on the wellbore. Treating these successivelyxe2x80x94i.e., with two different fluids, one after the otherxe2x80x94is problematic since, it requires at least two separate treatments. Combining two different breakers (one for the polymer fraction, one for calcite) is problematic since each has a distinct activity profile (or optimal window of activity, based on temperature, pH, etc.) and the activity profiles of two different breakers may not coincide. This is particularly likely if one of the breakers is an enzyme, which are notoriously temperature and pH sensitive.
Moreover, if the calcium carbonate is removed firstxe2x80x94as it often isxe2x80x94then, once the hydrochloric acid contacts the filtercake, regions of higher permeability are created in the wellbore (where the filtercake has dissolved). Hence, fluid will leak-off into the formation during subsequent phases of the filter-cake removal treatment.
Hence, the ideal fluid must be easy to xe2x80x9cspotxe2x80x9d or place in wellbore over the entire length of the desired zone, contiguous with the producing zone (e.g., a two thousand foot horizontal zone)xe2x80x94before any filtercake dissolution occurs. If the fluid begins to dissolve the filtercake too quickly, then the fluid will be lost through the thief zones and the entire fluid treatment will be destroyed. In other words, a hypothetical ideal fluid would be completely unreactive for a period of time to enable it to be spotted along the entire length of the producing interval, then, once in place, react sufficiently slowly and uniformly, so that no thief zones are. Again, if thief zones form, then the entire mass of fluid can leak off through that zone. Hence, reasonably uniform/controlled dissolution is necessary to ensure that the fluid remains in contact with the filtercake along the entire interval until near-complete dissolution of the filtercake has occurred along the entire interval.
Moreover, removing filtercake is an expensive and time-consuming procedure. Therefore, it is desirable to do this at the same time that another treatment is being performed, if possible. For instance, if a material must be delivered to one portion of the formation into the wellbore (e.g., in conjunction with a remedial treatment), then the fluid used to carry that material can be an acid solution which will also dissolve portions of the filtercake. Again, if the carrier fluid leaks off into the formation through a thief zone, then the remedial operation is completely destroyed.
One common treatment performed on wells, particularly wells in the Gulf Coast region of the United States, is known as a xe2x80x9cgravel pack.xe2x80x9d Gravel pack operations are performed to prevent the production of sand along with hydrocarbon, which often occurs in formations of weakly consolidated sands. To prevent sand production, a filter (or screen) can be placed around the portion of the wellbore in which production occurs. A more long-term solution for sand control is achieved if the region between the screen and the formation is filled with gravel, which is properly sized to prevent the sand from moving through the gravel and into the wellborexe2x80x94to function as a filterxe2x80x94so that when the sand tries to move through the gravel, it is filtered and held by the gravel or screen, but hydrocarbon continues to flow unhindered (by either the gravel or screen) into the wellbore.
Again, it would be highly advantageous if the fluid used to deliver the gravel could also be used to dissolve the filtercake, Which would eliminate the need for a separate treatment just to dissolve the filtercake. This would result in substantial cost savingsxe2x80x94both because a separate treatment is costly, and because it takes additional time to perform such a treatment.
Thus, what is desired is a fluid that can be used as a carrier fluid (though it need not be used for that purpose) and that can also degrade the filtercake. An ideal carrier fluid is inertxe2x80x94i.e., it should not degrade the filtercake instantaneously (otherwise the fluid can be lost into the formation)xe2x80x94but an ideal filtercake dissolution fluid must dissolve the cake, eventually. Therefore, an ideal fluid must somehow combine these two contradictory attributes.
Indeed, the need for filtercake clean-up is particularly acute in gravel pack completionsxe2x80x94i.e., wells in which the movement of sand along with the hydrocarbon is prevented by a gravel pack/screen combinationxe2x80x94because, the entrapment of the filter-cake between the formation and screens or gravel can result in substantial reduction in production. The need for a reliable filtercake clean-up treatment with a good diversion mechanism (to ensure proper placement) is also particularly acute in horizontal, or highly deviated wells. In these cases, the producing interval may be several thousand feet, compared with a vertical well, which may have a producing zone of about 30 feet. Because the difficulty of placing a mass of fluid to achieve near-uniform dissolution over 1000 feet interval is far greater than for a 30 feet interval-placement takes longer, and the potential for the creation of thief zones is far greater.
Therefore, an urgent need exists in the drilling and completions sector for a reliable fluid for degrading filtercakexe2x80x94quickly, efficiently, and completely, and which can be used as a carrier fluid in conjunction with other completion/workover/stimulation operations. This is the primary objective of the present Invention.
The present Invention relates to fluids intended to break filtercake (whether produced from drilling, production, completion, workover, or stimulation activity), either produced intentionally or unintentionally. U.S. patent application Ser. No. 09/224,440 in its entirety is incorporated herein by reference. In particularly preferred embodiments, the fluids and techniques are directed to degrading (or xe2x80x9cbreakingxe2x80x9d) filtercake formed from starch/carbonate-containing drilling fluid such as the STARDRILL(trademark) (a drill-in fluid manufactured and sold by Schlumberger). In other particularly preferred embodiments, the fluids of the present Invention are operable in conjunction with a gravel pack operation, and in particular, though not exclusively, to break filtercake, in conjunction with a gravel pack operation.
Therefore, one object of the present Invention is to provide novel completion fluids to break filtercake, either alone or in conjunction with other workover/completion/stimulation treatments, but in particular, gravel pack operations. Preferred embodiments relate to fluids to break filtercake having substantial calcite and starch content. Particularly preferred embodiments related to treatment fluids having two essential components: a chelating agent and an enzyme. These components were selected based on their ability to dissolve different components of the filtercake, and based on their ability to dissolve these components at particular rates relative to one another. Other particularly preferred embodiments are fluids having these two components in a VES (viscoelastic surfactant) system. VES systems have numerous advantagesxe2x80x94discussed at length in U.S. Patents incorporated by reference belowxe2x80x94including that they are readily gelled, they can be disposed of more easily than guar and modified guar systems, they are more readily removed from subsurface formations. In addition, and of particular importance of the present Invention, VES systems create very low friction pressures compared with conventional carrier fluids, and therefore they are particularly preferred, for instance, in gravel pack operations of the present Invention.
The fluids of the present Invention can be successfully spotted or placed over, for instance, a 2000 ft. horizontal producing zonexe2x80x94without substantial leakoff. Particularly preferred embodiments to achieve this incorporate Mobil""s AllPAC(trademark), (licensed exclusively to Schlumberger). This way, the gravel pack operation, for instance, can take place without fluid loss.
At the same time, the fluid of the present Invention acts slowly upon the filtercake, to slowly but steadily dissolve it, but not before the particular workover operation has been completed.
Moreover, the break time (or time to substantial dissolution of the filtercake) of the fluids of the present Invention are optimized so that the overall or blended dissolution rate is very slow at low temperatures but much higher at high temperatures. The primary advantage of this unique temperature-dependence is that fluid can be introduced into the entire zone of interest before filtercake dissolution occurs, then as the fluid temperature rises due to contact with the wellbore, only then does dissolution occur.
The fluids and techniques of the present Invention are quite general and are operable in a variety of settings. These include, but are not limited to, screen-only completions and gravel pack completions; open hole and cased hole; vertical and highly deviated wells; single-application soak or circulating fluid in which the treatment fluid (of the present Invention) also serves as a carrier fluid for, e.g., a gravel pack operation; in conjunction with a gelling agent or viscoelastic surfactant (e.g., ClearFrac(trademark)) or alone, and with a variety of clean-up tools. In summary, since the problem of placement and uniform dissolution are present in virtually every instance, the fluids and techniques of the present Invention are readily applicable to any scenario in which it is desirable to remove filtercake from the wellbore or near-wellbore region in the formation, regardless of whether the filtercake was produced during drilling or during other post-drilling operations (e.g., fluid-loss control pill, gravel pack operation, fracturing, matrix acidizing, and so forth).
Finally, the fluids of the present Invention are a viable, cost-effective replacement for HCl-based fluids, conventional fluids of choice to remove filtercake. Perhaps the major problem with HCl systems (aside from their ineffectiveness in removing the carbonate fraction of filtercake) is corrosionxe2x80x94corrosion of the above-ground storage tanks, pumps, down-hole tubulars used to place the fluid, and wellbore casings. Moreover, a cost-effective solution to corrosion is not readily available, as evidenced by the fact that corrosion inhibitors is a significant portion of the total expense of a filtercake removal (or matrix-acidizing) treatment. With many of the fluids of the present Invention (those that do not contain acid) the problem of corrosion is drastically minimized. Additionally, personnel safety and environmental concerns are significantly reduced with the fluids of the present Invention.
In one embodiment of the present invention, the fluid of the present invention comprises a viscoelastic surfactant having the formula CnCONHC3H6N+(CH3)2CH2COOxe2x88x92, wherein n ranges between 12 to 22, an enzyme (xcex1-amylase or xcex2-amylase) and a chelating agent. The chelating agent is preferably selected among 1-hydroxyethylidene-1, 1-diphosphonic acid-1 (HEDP); aminotri(methylene phosphonic acid) (ATMP); ethylenediaminetetraacetic acid (EDTA); cyclohexanediaminetetraacetic acid (CDTA); diethylenediaminepentaacetic acid (DPTA); nitrilotriacetic acid (NTA); hydroxyethylethylenediaminetriacetic acid (HEDTA); and hydroxyethyliminodicetic acid (HEIDA.